1. Field of the Invention
The present invention relates generally to apparatus for depositing coatings on substrates and, more particularly, is concerned with an improved cathode/ground shield arrangement in a target of an apparatus for sputter coating nuclear fuel pellets.
2. Description of the Prior Art
Conventional nuclear fuel pellets commonly have the shape of generally right circular cylinders and contain fissionable material, such as uranium dioxide, thorium dioxide, plutonium dioxide, or mixtures thereof. The pellets are placed in end-to-end abutment within a cladding tube (typically made of a zirconium alloy or stainless steel) to make a fuel rod, and an array of such fuel rods are grouped together to form a fuel assembly. A plurality of fuel assembies are arranged together to constitute the core of a nuclear reactor.
As generally well-known, the life of a fuel assembly may be extended by combining an initially larger amount of fissionable material with a calculated smaller amount of burnable absorber. Burnable absorbers are materials (such as boron, gadolinium, samarium, europium, and the like) which have a high probability (or cross section) for absorbing neutrons, resulting in isotopes of sufficiently low neutron capture cross section so as to be substantially transparent to neutrons, all while producing no new or additional neutrons. The burnable absorber compensates for the larger amount of fissionable material during the early life of the fuel assembly. During reactor operation, the burnable absorbers are progressively reduced in amount, resulting in a longer life at relatively constant fission level for the fuel assembly. Longer fuel assembly life means less frequent nuclear reactor fuel assembly replacement. This refueling process is costly and time consuming.
Sputtering as a method for depositing a layer of burnable poison on nuclear fuel pellets is disclosed in the first patent application cross-referenced above. The second cross-referenced patent application discloses an apparatus for accomplishing the sputtering operation. Other sputtering apparatuses for depositing coatings on other types of products are disclosed in U.S. Pat. Nos. 3,562,140, 3,632,494 and 4,080,281.
In a typical sputter process, wherein a coating is deposited on a substrate, an inert gas is introduced into a vacuum chamber containing an anode, the substrate, and a target cathode. A high voltage is applied across the anode and the target cathode. The gas molecules are ionized and strike the target cathode. This causes atoms and/or molecules of the target cathode to be sputtered from the target cathode's surface. The substrate is positioned to intercept this sputtered target material which forms an adherent coating upon striking the substrate. Sputtering is a generally line-of-sight coating process.
In the apparatus of the second cross-referenced application, each of a plurality of targets includes a cathode and target material in the form of a plurality of rectangular tiles of zirconium diboride positioned on the top surface of the cathode. The cathode is usually made of copper with magnetic material inside which concentrates electrons in the vicinity of the cathode. This concentration enhances the generation of argon ions which collide with the negatively charged target material setting on the top surface of the cathode. However, the other surfaces of the cathode must be protected from the argon ions to keep from sputtering the material of the cathode itself.
Consequently, in the sputtering apparatus of the second cross-referenced application, each target also employs a ground shield used to protect these other cathode surfaces. The ground shield is positively charged, and therefore collects electrons and repels the argon ions. As shown in FIGS. 7 and 8 herein, the ground shield is physically located so that a narrow gap, for instance one-eighth to three-sixteenth of an inch, exists between the shield and the cathode. In this arrangement the ground shield is far enough away from the cathode to prevent an electric arc from occurring between them under normal conditions of relative humidity but close enough to prevent a plasma from starting because the electrons will have insufficient energy to ionize the argon.
However, problems have been recently discovered with the above-described arrangement of the ground shield and cathode of the target. Relative humidity in the sputter coating facility is frequently higher than normal because of high water vapor loads present from outgassing of the porous target tiles and from the nuclear fuel pellets being coated. High relative humidity has produced arcing and shorting of the cathodes used in the facility, occurring in the cathode to ground shield gap located on all sides of the cathode. These arcs are, at times, so severe that penetration of water cooling channels in the ground shield occurs, thus causing a water leak. A flake of zirconium diboride emanating from a location above the target material will land across the gap so as to cause a short circuit to arise between the oppositely charged ground shield and cathode. Such short circuit necessitates shutting down the coating apparatus, resulting in significant expense and loss of time. Consequently, a need exists for improvement of the sputtering apparatus to eliminate its susceptibility to occurrence of arc and short circuit conditions.